Once someone agrees to speak, do not respond with only, “When are you free?” That makes the other person do all the coordination. Instead, include several specific windows when you are available, along with your time zone.
Offer options across at least two days when possible, and make the windows easy to scan. For example: “I am available Tuesday from 10:00 a.m. to noon ET, Wednesday from 2:00 to 5:00 p.m. ET, or Thursday after 4:00 p.m. ET.” Keep enough flexibility around each window to accommodate a 15- to 20-minute conversation.